tourniquet modeling, analysis and physical testing

Blew Consulting, LLC

A report submitted to:

HALO Tactical Products, LLC 13728 Statesville Rd Huntersville, NC 28078 Attn: Daniel P. Cedrone Prepared by:

Douglas J. Blew, PE 135 Sienna Lane Mooresville, NC 28117 Phone: +1.828.244.0557 [email protected]

Testing Dates:

February 18 & 19, 2019

April 22, 2019

November 19, 2019

Final Report Date:

April 27, 2020

Disclaimer: This report was prepared by Blew Consulting, LLC for HALO Tactical Products, LLC. The material herein reflects Blew Consulting, LLC’s best judgment given

the information available at the time of preparation. Any use which a third party makes of this report, any reliance on or decision to be made based on it, are the

responsibility of such third parties. Blew Consulting, LLC accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or

actions based on this report.

Tourniquet Modeling, Analysis and Physical Testing

Abstract

This report joins and summarizes a series of prior reports where modeling,

analytical methods and physical testing were undertaken to validate

strength requirements and determine safety factors of a mechanical

battlefield applied tourniquet manufactured by HALO Tactical Products, LLC

(the Client).

In this document:

1. Executive Summary

2. Client Input / Testing Criteria

3. Modeling and Validation of Strength Criteria

4. Tourniquet Component Physical Testing

5. Complete Assembled Tourniquet Physical Testing

6. Appendix A - Detailed Test Data

7. Appendix B - Equipment List and Calibration

mailto:[email protected]

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1. Executive Summary

HALO Tactical Products, LLC contracted Blew Consulting, LLC to perform a series of mechanical strength tests on their

HALO® battlefield applied tourniquet and determine resultant margins of safety for critical individual components and

the tourniquet as a whole.

To calculate factors of safety (FoS) from strength values obtained from physical testing, minimum strength requirements

had to be either provided (known) or determined from criteria based on tourniquet application and use. As there were

no known strength values available, a study was undertaken to determine the maximum tensile strength the HALO®

tourniquet had to withstand based on established medical efficacy for inflatable surgical tourniquets. With the strength

requirement determined, all physical strength-testing results could then simply be divided by the required strength to

determine FoS.

HALO Tactical Products’ medical professional1 provided the following criteria to drive all simulation, analysis and testing:

- Maximum limb occlusion pressure (LOP) of 350 mmHg as referenced to a standard 3 inch wide inflatable surgical

tourniquet.

- 95th percentile adult male thigh size corresponding to a 28.13 inch circumference / 8.95 inch diameter.

Utilizing classical hoop-stress equations, computer finite element analysis and physical testing, it was determined that

an inflation pressure of 350 mmHg in a 3 inch wide surgical tourniquet resulted in a maximum cuff or circumferential

tension of 91 lbf. Utilizing the same calculation methods applied to the narrower width (1.71 inch) HALO® tourniquet

resulted in a circumferential tension of 52 lbf with the 350 mmHg simulated internal pressure. The reduction in

circumferential tension is a result of and directly proportional to the cuff area as determined by difference in width

between the two tourniquets.

Medical input and literature1, 2, 3, 4 indicates that narrower tourniquets actually require higher radial inward unit

pressures [pneumatically or mechanically generated] to occlude blood flow. As such, the maximum radial inward unit

pressure for the narrower (1.71 inch) HALO® tourniquet increases by an experimentally determined “Limb Occlusion

Formula3, 4” to a maximum of 536 mmHg to achieve the same efficacy as a 3.0 inch inflatable surgical tourniquet

operating at 350 mmHg. At this pressure, the resulting tension in the cuff of the 1.71 inch HALO® tourniquet is 80 lbf.

For both tourniquet component and complete tourniquet testing, sets of 5 articles each were tested at room

temperature, -20°F and +120°F. Based on the 80 lbf maximum tensile load occurring at the maximum tourniquet

applied pressure, the minimum average FoS for the individual components was 3.7.

When testing the entire tourniquet, as deployed around a simulated 95th percentile limb, the minimum average FoS was

4.5.

For a single use mechanical device, the Author recommends a minimum FoS of 1.5 (50% stronger than a worse-case

expected load) as appropriate.

1 Dr. William R, Carson, MD

2 J. A. McEwen, V. Casey. Measurement of hazardous pressure levels and gradients produced on human limbs by non-pneumatic tourniquets, (2009). CMBEC32.

Calgary, Canada; 2009 May 20-22. 3 Brent Graham, M.D., F.R.C.SC et al. Occlusion of Arterial Flow in the Extremities at Subsystolic Pressures Through the Use of Wide Tourniquet Cuffs. Clinical

Orthopaedics and Related Research; Number 286 - January 1983 4 COL John F. Kragh Jr., MCUSA et al. The Military Emergency Tourniquet Program’s Lessons Learned With Devices and Designs. Military Medicine, 176 10:1144, 2011

https://tourniquets.org/wp-content/uploads/OurLiteraturePDFs/McEwen-Measurement-of-hazardous-pressure-levels-and-gradietns-produced-on-human-limbs-by-non-pneumatic-tourniquets.pdf

https://tourniquets.org/wp-content/uploads/OurLiteraturePDFs/McEwen-Measurement-of-hazardous-pressure-levels-and-gradietns-produced-on-human-limbs-by-non-pneumatic-tourniquets.pdf

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2. Client Input / Testing Criteria

The Client requested the following:

1. An analysis to determine how inflation pressure applied to an inflatable medical surgical tourniquet correlates to

the resultant circumferential tensile forces within the cuff of a mechanically actuated tourniquet (or similar

device).

2. Physical tensile strength testing of various sub-components of Client’s tourniquet [HALO®] at room temperature,

-20°F and +120°F.

3. Physical tensile strength testing of the complete and fully deployed tourniquet at room temperature, -20°F and

+120°F.

For the correlation of inflation pressure to tensile force within the cuff, the Client provided a limit of 350 mmHg inflation

pressure as an upper testing limit for Limb Occlusion Pressure (LOP). LOP is the point where the applied pressure in a

tourniquet stops blood flow to a limb extremity beyond the applied tourniquet.

For the sub-component testing, the Client prepared individual samples attached to sewn loops sufficient to be

suspended between test-pin tooling of a tensile testing machine.

The Client stipulated and all analysis and testing of complete (whole) tourniquets were carried out on a simulated “limb”

sized for a 95th percentile adult male thigh (28.13 inch circumference). The corresponding diameter of 8.95” [diameter =

circumference / π] was used as input to calculations, computer simulations and the design fixturing for the physical

testing.

All tourniquets and other test specimens were supplied by the Client. All test fixtures and ancillary devices were

designed or provided by Blew Consulting, LLC. All physical testing took place at an independent lab by qualified testing

personnel.

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3. Modeling and Validation of Strength Criteria

When designing a part or an assembly (a series of joined parts), one must assure that there is no weak link in the

assembly which might fail in the course of normal use. It is further customary to overdesign components for

unanticipated situations, degradation or abuse by including a factor of safety (FoS) in the design criteria for strength,

fatigue, etc. There is no universal FoS, as safety levels are normally determined by type of industry and/or levels of

acceptable risk, but usually range from a low of 1.25 to a high of 4. Weight-critical designs utilized in aerospace typically

use a lower FoS, but compensate with extreme levels of quality control. The higher FoS criteria are employed for

applications such as pressure vessels and nuclear. Most building design utilizes a FoS between 1.75 and 2 for structural

members. In all cases, the design stress is determined, multiplied by the FoS and then the resulting value of stress is

used to ultimately design the component. If a design is already in existence, the FoS is determined by testing the design

for its strength and then dividing the strength of the components by the maximum design load / stress.

In the case of the subject mechanical tourniquet, there was no design stress level or tensile load requirement provided

by the Client – just the maximum LOP of 350 mmHg utilized in established inflatable surgical tourniquets. To determine

the required strength criteria for the mechanical tourniquet which corresponded to 350 mmHg of pressure applied to an

inflatable tourniquet, the Author chose to approach the problem via multiple methods:

First, fundamental and well-established design equations were applied to the problem.

Second, computer simulations were employed via Finite Element Analysis.

Last, actual physical lab tests were conducted utilizing an inflatable tourniquet.

All approaches are compared and then a “Limb Occlusion Formula3, 4” is used to apply the data obtained in the physical

testing of the inflatable tourniquet to the predicted results of the mechanical [HALO®] tourniquet.

CLASSICAL EQUATIONS

For thin-walled cylinders under a known uniform pressure [P], if the radius [r] and wall thickness [t] are known, the hoop

stress [ơh] can be determined by the formula:

ơh = P*r/t

The equation is a derivation of the Lame Equation for general hoop stress and is valid for cases where the diameter to

thickness ratio is greater than 20 (which is true in this case). Units can be in SI with Pressure denoted as Pascals and r

and t as meters or in Imperial units with Pressure in psi and r and t in inches.

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RESULTS

To determine the cross-sectional area of the strapping for the inflatable and mechanical tourniquets, the following

measured widths and assumed thicknesses were used:

Inflatable (actual width = 3.00”, thickness = 0.080”) Mechanical (actual width = 1.72”, thickness = 0.080”) For the thickness of the tourniquets, we can use an assumed thickness instead of exact values – as we are only

interested in the tensile forces generated and not the particular unit stress levels. For a given unit stress (psi), the area

(in the denominator) is simply canceled out when multiplied by the actual cross-sectional area to provide the tensile

force for a given strap width (it washes out). Thus, for a given stress level, the tensile force (lbf) in the strapping is

directly proportional to the width of the strap and has no relation to the strap thickness and the equation reduces to:

Force = ơh*Area The following table and graph show the results of the calculations (with all mmHg values converted to psi).

Pressure (mmHg)

Pressure (psi)

Stress (psi)

Inflatable Tensile (lbf)

HALO Tensile (lbf)

100 1.93 108.2 26.0 14.9

125 2.42 135.2 32.4 18.6

150 2.90 162.2 38.9 22.3

175 3.38 189.3 45.4 26.0

200 3.87 216.3 51.9 29.8

225 4.35 243.4 58.4 33.5

250 4.83 270.4 64.9 37.2

275 5.32 297.5 71.4 40.9

300 5.80 324.5 77.9 44.7

325 6.28 351.5 84.4 48.4

350 6.77 378.6 90.9 52.1

375 7.25 405.6 97.3 55.8

400 7.73 432.7 103.8 59.5

0.0

20.0

40.0

60.0

80.0

100.0

120.0

100 125 150 175 200 225 250 275 300 325 350 375 400

Ten

sile

fo

rce

(lb

f)

Pressure (mmHg)

Pressure -vs- Strapping Tensile Force

Inflatable Mechanical - HALO

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Finite Element Analysis Computer Simulations Computer simulation via Finite Element Analysis (FEA) can be used to accurately solve much more complex situations

than the simple reduced thin-wall Lame approximations. Models of the part(s) must be generated, material properties,

constraints and loads applied and then simulations can be executed.

METHOD

SolidWorks Professional 2018 was used to model 90-degree (1/4) segments of both the inflatable and mechanical

(HALO®) tourniquets. SolidWorks Simulation was then used to apply constraints to the models, apply internal pressure

and then solve for the resultant strains and loads. The setup dictates that only 90-degree bisected segments of the

whole model in the pure ‘X’ and pure ‘Y’ directions are required in order to capture the principle strains. The only

difference between the two models (inflatable vs mechanical – HALO®) was the width. Internal pressures were

simulated at 100 mmHg, 200 mmHg, 300 mmHg and 400 mmHg for each tourniquet type.

HALO® Model Inflatable Model

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RESULTS

Inflation Pressure (mmHg)

Tensile Force (lbf) Inflatable

Tensile Force (lbf) HALO®

100 25.9 14.9

200 51.8 29.7

300 77.8 44.6

350 (interpolated) 90.8 52.1

400 103.7 59.5

DISCUSSION / OBSERVATIONS / CONCLUSIONS

The 8 simulations solved in reasonable amount of time and generated results that highly agreed with the manual (Lame)

calculations.

0

20

40

60

80

100

120

100 200 300 400

Ten

sile

fo

rce

(lb

f)

Pressure (mmHg)

Pressure -vs- Strapping Tensile Force FEA Simulation Results

Inflatable Mechanical (HALO)

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Tourniquet Inflation Pressure -vs- Tensile Load Test It was theorized that test fixtures and components could be designed / fabricated / acquired such that tourniquet

inflation pressure could be readily correlated to the resultant tensile load imparted to the circumferential strap

surrounding the device by direct measurement.

METHOD

A fixture was constructed with the desired diameter (8.95”) and then separated into two halves (mandrels). The

mandrels were mounted into a tensile tester (Instron), positioned approximately 1/4” apart, an inflatable tourniquet

was applied around the exterior of the mandrels, the tourniquet was hooked up to a medical inflation device and then

deployed (inflated) in 25 mmHg increments from 100 mmHg to 400 mmHg. As the tourniquet was inflated, the resulting

tensile force between the two mandrel halves was measured. To check for any hysteresis effects, once the pressure

reached maximum (400 mmHg), the pressure was reduced back to 100 mmHg in 25 mmHg increments and also

recorded.

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RESULTS

Tabular and graphical results are shown below.

0

50

100

150

200

250

100 125 150 175 200 225 250 275 300 325 350 375 400

Ten

sile

fo

rce

(lb

f)

Pressure (mmHg)

Pressure -vs- Strapping Tensile Force

Series1 Series2 Series3 Series4


Tensile Load (lbf) Up [Total]

Tensile Load (lbf) Down [Total]

Tensile Load (lbf) Avg [Total]

Tensile Load (lbf) [per leg]

100 45 45 45.0 22.5

125 59 60 59.5 29.8

150 69 70 69.5 34.8

175 83 82 82.5 41.3

200 96 93 94.5 47.3

225 106 106 106.0 53.0

250 119 118 118.5 59.3

275 130 130 130.0 65.0

300 143 141 142.0 71.0

325 155 154 154.5 77.3

350 169 165 167.0 83.5

375 178 175 176.5 88.3

400 191 186 188.5 94.3

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The force generated between the mandrels as a result of the inflation pressure behaved linearly. The values obtained

during inflation and deflation matched very well, indicating there was little hysteresis effect. The actual forces in the

tourniquet cuff are obtained by dividing the mandrel tensile force by 2. This is because there are two “legs” of the

tourniquet cuff that are resisting the forces exerted between the mandrels. This is illustrated by the following free-body

diagram that must be in balance:

Summary / Comparative Results


Inflatable Calculated (lbf)

Inflatable FEA (lbf)

Inflatable Actual (lbf)

HALO® Calculated (lbf)

HALO® FEA (lbf)

100 26.0 25.9 22.5 14.9 14.9

125 32.4 29.8 18.6

150 38.9 34.8 22.3

175 45.4 41.3 26.0

200 51.9 51.8 47.3 29.8 29.7

225 58.4 53.0 33.5

250 64.9 59.3 37.2

275 71.4 65.0 40.9

300 77.9 77.8 71.0 44.7 44.6

325 84.4 77.3 48.4

350 90.9 90.8 83.5 52.1 52.1

375 97.3 88.3 55.8

400 103.8 103.7 94.3 59.5 59.5

0

20

40

60

80

100

120

100 125 150 175 200 225 250 275 300 325 350 375 400

Ten

sile

fo

rce

(lb

f)

Pressure (mmHg)

Modeling, Pressure -vs- Strapping Tensile Force

Cacl Inflatable FEA Inflatable Acutal Inflatable Calc HALO FEA HALO

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As previously noted, the manually calculated values (Lame) and finite element computer analyses highly agree with each

other for both tourniquet types. On average, for the inflatable tourniquet cases, there is a -10% difference between the

forces generated by actual inflation pressures in the physical tourniquet and the analytical results. This is a result of

non-linear forces across the width of the tourniquet and unavoidable inefficiencies as the inflatable bladder must work

against and overcome all of its constraining elements. Constraining elements for the bladder include the elastic forces

required to inflate the bladder, frictional forces between the bladder and the surrounding materials, and slight

pneumatic losses in the system.

The difference in strap (cuff) tensile force -vs- applied pressure between the inflatable tourniquet and the manually

applied tourniquet (HALO®) is entirely due to the difference in contact surface area between the two as determined by

the effective strap (cuff) widths. This is due to the application of pressure (stress) over differing areas…. [recalling earlier

that Force = stress * area].

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Correlation Between Inflatable and Mechanical Tourniquets

It has been previously shown that the resulting tensile force in an inflatable tourniquet strap (cuff) is not only

proportional to the inflation pressure, but also to the overall area that the pressure is applied over, as determined by

strap (cuff) width or strap area. In comparing tourniquets of different widths and differing mechanisms for generating

circumferential pressure [inflatable vs mechanical], medical research has been conducted that has resulted in the

following equation where LOP is determined as a function of the ratio of limb circumference to cuff width4,5.

LOP = (limb circumference / cuff width) 16.67 + 67

Substituting the cuff widths of the 3.0 inch inflatable surgical tourniquet and the 1.71 inch HALO® tourniquet in

combination with a 95th percentile limb circumference (28.13 inch), yields the following results:

Tourniquet Type Tourniquet Width (Inches)

Minimum Limb Occlusion Pressure (mmHg)

Inflatable Surgical 3.0 223

HALO® 1.71 341

At the minimum occlusion pressure, the effective pressure ratio between tourniquets is 1.53; thus the 1.71 inch

tourniquet requires 53% more pressure applied over its reduced area. For the maximum design pressure requirement of

350 mmHg for the normative inflatable tourniquet, the 1.71 inch HALO® tourniquet then correlates to 536 mmHg (1.53 *

350 mmHg).

From previously determined work, 536 mmHg acting over a 1.71 inch cuff can be linearly extracted to a cuff force (in

tension) of 79.7 lbf.

Based on the above, a maximum design tensile force of 80 lbf will be used in calculating factors of safety results

stemming from the physical testing of the actual tourniquets and tourniquet components.

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4. Tourniquet Component Physical Testing

INTRODUCTION

The Client requested tensile strength testing of specimens constructed from various isolated portions of the tourniquet

assembly. To facilitate testing, the Client prepared all samples with two sewn connection loops fashioned at the end of

each article. The loops were sufficient to slide over 5/8” mandrels attached to the test fixture which was in turn

attached to the tensile testing machine [Instron]. The temperatures requested were -20°F, 65°F – 75°F [room temp] and

+120°F. 5 samples were provided for testing at each temperature. The samples provided were not identifiable as to

model number, lot number or manufacturing date, but were all stated as being configurations representative of pre-

production samples.

METHOD

All samples were conditioned at their target temperature for a minimum of 2 hours. Room temperature samples were

simply left exposed within the lab room environment while the +120°F and -20°F samples were placed into calibrated

temperature-controlled chambers.

The 5/8” mandrels were pre-positioned at the approximate loop separation distance and the loops of the

subcomponents were slid around the mandrels. Each hot and cold sample was individually brought out of its controlled

environment, immediately wrapped in insulative material and then deployed on the test apparatus as quickly as possible

to preserve the article temperature. Once the article was in position, the tensile test was initiated at a rate of 2” /

minute and continued until the article under test failed. The maximum load [lbf] at failure and the failure mode was

recorded. All samples were returned to the Client after testing was completed.

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HALO® Main Strap Tensile Strength at 3 Different Temperatures

Tabular results are shown below. The individual stress-strain graphs are included in the appendix.

Test Sample Temp [°F] Result [lbf] Mode of Failure

1 Room 301.8 Failed at outer edge of single-stitched loop

2 Room 292.5 Failed at inner edge of double-stitched loop

3 Room 285.0 Failed at inner edge of single-stitched loop



Average 299.0

Std Dev 10.1


1 -20 302.9 Failed at inner edge of single-stitched loop



4 -20 324.2 Failed ~ 50% at both edges of stitched loop


Average 316.4

Std Dev 14.1


1 +120 275.1 Failed at inner edge of single-stitched loop




5 +120 298.2 Failed at inner edge of double-stitched loop

Average 296.3

Std Dev 28.9

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All the specimens failed in pure tensile at one edge or the other of the sewn area where the loops are joined back into

the main strapping material. All but one sample failed at the inner edge of the sewn strap (facing towards the center of

the test sample). 4 of 15 samples failed at the double stitched interface; however it should be noted that this loop was

consistently formed such that the active “hook” material was adjoining each other at the sewn interface. Conversely, all

the single sewn interfaces had the smoother “loop” sides contacting each other and exhibited no latching effect at the

overlapping sewn interface. The points of failure indicate that the punctures in the material due to stitching reduce the

effective strap area and/or create stress concentration areas that bias the failures to those areas.

The average of all results had 20.1 lbf variation between them, with the +120°F sample set having the most significant

variation when standard deviations were taken into account.

At the maximum tensile force of 80 lbf, the resulting testing indicates a minimum FoS of 3.7 occurring at +120°F.

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HALO® Locking Strap Tensile Strength at 3 Different Temperatures

RESULTS



1 Room 225.3 Failed between loop stitching and buckle stitching

2 Room 236.7 Same as above




Average 227.2

Std Dev 9.7


1 -20 230.4 Failed between loop stitching and buckle stitching

2 -20 230.4 Same as above




Average 226.0

Std Dev 5.8


1 +120 218.4 Failed between loop stitching and buckle stitching

2 +120 232.3 Same as above




Average 229.5

Std Dev 7.0

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All the specimens failed in pure tensile within the single width of material between the two stitched loops. More

specifically, the failures were all immediately adjacent to one or the other of the stitched areas. The points of failure

indicate that the punctures in the material due to stitching reduce the effective strap area and/or create stress

concentration areas that bias the failures to those areas.

Comparing the averages between temperature groups yields only 3.5 lbf variation with standard deviations all below 10

lbf. Unlike the main strap, this locking strap is responsible for actuating and securing the lever action of the buckle and

is mechanically subjected to less tensile stress than the main strap during deployment. Based on measurements of the

buckle, the mechanical advantage provided by the geometry is approximately 3.3x. With a main strap maximum tensile

force of 80 lbf, the force required to achieve this from the locking strap would be reduced by a factor of 3.3 to 24.2 lbf,

resulting in a minimum average FoS of 9.3 occurring at -20°F for the locking strap material. It must be kept in mind,

however, that the actual retention of the strap in a stationary position after deployment relies on a “hook and loop”

material attachment to hold the buckle in a deployed position. This is evaluated in the following section when testing

the entire tourniquet.

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HALO® Locking Buckle with Main Strap Material Tensile Strength at 3 Different Temperatures

RESULTS



1 Room 256.5 Failed between loop stitching and buckle stitching





Average 299.2

Std Dev 31.5


1 -20 314.4 Failed between loop stitching and buckle stitching





Average 317.5

Std Dev 17.7


1 +120 274.9 Failed between loop stitching and buckle stitching





Average 311.1

Std Dev 40.5

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All the specimens failed in pure tensile within the single width of material between the two stitched loops. More

specifically, the failures were all immediately adjacent to one or the other stitched areas. The points of failure indicate

that the punctures in the material due to stitching reduce the effective strap area and/or create stress concentration

areas that bias the failures to those areas.

Comparing the averages between temperature groups yields 18.3 lbf variation with standard deviations ranging from

17.7 to 40.5. The main strap construction has “hook” material on one side and “loop” material on the opposite side. In

the formation of the loops, the “loop” sides formed the inside of the test loop and as such were sewn facing each other.

It has been observed in testing that adjoining “hook-loop” and “hook-hook” surfaces results in adhesion, while this is not

true for the “loop-loop” orientation represented in these articles under test. Based on observed test behavior, the

author believes the alternate orientations may slightly improve the resulting tensile strength of the joint.

At the maximum tensile force of 80 lbf, the resulting testing indicates a minimum FoS of 3.7 occurring at 68°F.

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5. Complete Assembled Tourniquet Physical Testing

INTRODUCTION

The Client requested tensile strength testing of a deployed HALO® tourniquet applied around a mandrel sized for a 95th

percentile adult male thigh (28.13 inch circumference - further provided by the Client). The corresponding diameter of

8.95” [diameter = circumference / π] was used to design fixturing for the tensile test. Two identical mandrel halves were

designed and fabricated to hold the tourniquet and to interface with the tensile testing machine [Instron]. The

temperatures requested were -20°F, 65°F – 75°F [room temp] and +120°F. 5 samples were provided for testing at each

temperature. The samples provided were not identifiable as to model number, lot number or manufacturing date, but

were stated as being pre-production samples.

METHOD

All samples were conditioned at their target temperature for a minimum of 2 hours. Room temperature samples were

simply left exposed within the lab room environment while the +120°F and -20°F samples were placed into calibrated

temperature-controlled chambers.

The mandrel halves were positioned approximately ¼” apart, the tourniquets were wrapped around the exterior of the

mandrels and then deployed (tightened) according to the manufacturer’s instructions. The additional “hook and loop”

“finger loop locking tab” strap was evenly positioned such that it additionally secured the distal end of the main strap

where it is affixed back onto itself as shown below.

Hot and cold samples were individually brought out of their controlled environments, immediately wrapped in insulative

material and then deployed on the test apparatus as quickly as possible to preserve the article temperature. Once

deployed, the tensile test was initiated at a rate of 2” / minute and continued until the article under test failed due to

either breakage or slippage of the securing straps. The maximum load [lbf] at failure and the failure mode were

recorded. All samples were returned to the Client after testing was completed.

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RESULTS


1 Room 768.3 Release of “hook and loop” main strap in shear

2 Room 671.1 Main strap broke at sewn buckle end

3* Room 395.8* Test error, test repeated with new sample

4 Room 734.0 Release of “hook and loop” main strap in shear

5 Room 763.2 Main strap broke mid-span

6* Room 717.8 Additional sample tested, strap broke at buckle end

Average 730.9* *Sample 3 eliminated from data

Std Dev 39.4* *Sample 3 eliminated from data


1 +120 832.0 Main strap broke mid-span

2 +120 623.1 Release of “hook and loop” main strap in shear




Average 727.3

Std Dev 74.4


1 -20 741.9 Main strap broke mid-span


3 -20 707.0 Release of “hook and loop” main strap in shear



Average 754.5

Std Dev 31.7

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The actual forces in the strapping are obtained by dividing the mandrel pulling tensile force by 2. This is because there

are two “legs” of the tourniquet that are resisting the forces exerted between the mandrels. This is illustrated by the

following free-body diagram:

The tourniquets failed in 3 different modes, with an overall range of 623 lbf – 832 lbf. In 7 of the tests, the “hook and

loop” interface of the main strap (with the “finger loop locking tab” attached) released gradually via shear. In 6 of the

tests, the main strap broke in tensile near the middle of the tourniquet. The remaining 2 failures were at the stitching

where the main strap is sewn back onto itself after wrapping around the buckle.

The results at +120°F showed more variation than the other sample groups with the highest load [832.0 lbf] and lowest

load [623.1 lbf] and a corresponding standard deviation of 39.4 lbf. The -20°F group had the highest average [754.5 lbf]

and also the narrowest standard deviation of 31.7 lbf. The Author surmises that the yield strengths of the interlocked

plastic “hooks” and “loops” decrease with increasing temperature and result in an earlier release at the elevated

condition.

At the maximum tensile force of 80 lbf, the resulting testing indicates a minimum FoS of 4.5 occurring at +120°F.

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6. Appendix A - Detailed Test Data

0

50

100

150

200

250

300

350

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Load

(lb

f)

Extension (in)

HALO® Main Strap Tensile Strength - Room Temp

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0

50

100

150

200

250

300

350

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Load

(lb

f)

Extension (in)

HALO® Main Strap Tensile Strength -20°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0

50

100

150

200

250

300

350

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Load

(lb

f)

Extension (in)

HALO® Main Strap Tensile Strength +120°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

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0

50

100

150

200

250

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

Load

(lb

f)

Extension (in)

HALO® Locking Strap Tensile Strength - Room Temp

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0

50

100

150

200

250

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Load

(lb

f)

Extension (in)

HALO® Locking Strap Tensile Strength - 20°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0

50

100

150

200

250

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Load

(lb

f)

Extension (in)

HALO® Locking Strap Tensile Strength + 120°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

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0

50

100

150

200

250

300

350

0.00 0.50 1.00 1.50 2.00 2.50

Load

(lb

f)

Extension (in)

HALO® Locking Buckle w/ Main Strap Material Tensile Strength - Room Temp

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0

50

100

150

200

250

300

350

400

0.00 0.50 1.00 1.50 2.00 2.50

Load

(lb

f)

Extension (in)

HALO® Locking Buckle w/ Main Strap Material Tensile Strength - 20°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0

50

100

150

200

250

300

350

400

0.00 0.50 1.00 1.50 2.00 2.50

Load

(lb

f)

Extension (in)

HALO® Locking Buckle w/ Main Strap Material Tensile Strength + 120°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

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0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

Load

(lb

f)

Extension (in)

HALO® Complete, Loop Tensile - Room Temp

Sample 1

Sample 2

Sample 4

Sample 5

Sample 6

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

Load

(lb

f)

Extension (in)

HALO® Complete, Loop Tensile +120°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

Load

(lb

f)

Extension (in)

HALO® Complete, Loop Tensile -20°F

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

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7. Appendix B - Equipment List and Calibration

1. Instron Tensile Tester

Model #: 5567

Serial #: 5567-C5428

Load Cell: 2525-810, Serial #: 132

2. Stryker Color Cuff®

Size: 34 in

Model #: 5921-034-701

3. SCANDMED Tourniquet Inflation System

Model #: 400-40

Serial #: 8120

Serviced: 8/2018

Due: 9/2020

4. Cincinnati Sub-Zero Environmental Chamber

Model #: ZP-32-3.5-SCT/AC

Serial #: ZP0711198

5. Fisher Scientific Recirculating Oven

Model #: 725F

Serial #: 1584061011161

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tourniquet modeling, analysis and physical testing

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